Conventional manufacturing of wet-process phosphoric acid, as for fertilizer production, sets up extensive acid pond water cooling ponds and gypsum stacks, often measuring several hundred acres each. Authorities include A. V. Slack, ed. Phosphoric Acid (Decker 1968); and P. Becker PHOSPHATES AND PHOSPHORIC ACID--Raw Materials, Technology, and Economics of the Wet Process (Decker 1983), especially chapter 9 thereof, entitled "What to Do with Gypsum" (pp. 471-496).
Wherever in contact with the ground, resulting phosphate acid pond (or "gyp pond") waters pollute the ground and the ground water with residual phosphoric acid and soluble fluorides, also dissolved metal impurities and radioactive compounds. Underlying impermeable barriers are required to contain such pollutants, but impermeability over the long term is very difficult and expensive to sure. Regardless of the presence or effectiveness of an underlying barrier, gyp pond waters pollute the air by emitting fluoride-containing gases. The liquid itself is extremely acidic (pH from about 1.8-2.0), being composed of weak phosphoric acid ranging from 1.3-2.5% P.sub.2 O.sub.5, fluosilicic and soluble fluorides ranging from 0.5-1.8%, sulfuric acid and soluble sulfates ranging from 0.5-1.6%, plus a number of soluble metal salts and radioactive compounds from the wet-process, all regarded by environmental authorities (and by us) as pollutants.
Attempts to limit the resulting contamination have had modest degrees of success, but acceptance of the ponds as a necessary evil has precluded comprehensive treatment of the underlying pollution, and fundamentally has precluded its satisfactory resolution. Hence, for the most part the attempts have been constrained or frustrated by the self-limiting condition that the acid ponds are here to stay.
Examples in U.S. patents include treatment of acid pond waters to remove metallic ions, as in Mills U.S. Pat. No. 4,303,532; to recover calcium fluoride, as in Hirka & Mills U.S. Pat. No. 4,171,342 or O'Neill U.S. Pat. No. 4,374,810; to remove calcium fluoride to enable use in wet ball-milling of phosphate rock, as in O'Neill et al. in U.S. Pat. No. 4,472,268; and to remove ammonia, as in Zibrida U.S. Pat. No. 4,698,163.
The present inventors, individually and jointly, have developed many improvements in the phosphate art, recognized in the following U.S. Pat. No. 3,699,212 (Palm); U.S. Pat. Nos. 3,720,757 and 3,859,423 (Hartig); and U.S. Pat. No. 4,320,012 (both). The latter joint patent teaches two-stage neutralization of such acid waters, with underflow from the second stage fed back into the first stage--contrary to a simpler two-stage treatment, as by Randolph in U.S. Pat. No. 3,625,648, for example.
More recently the present inventors developed the inventions of the applications identified above (whose priority is claimed here) to extend the benefits achieved by their previous two-stage modification of the wet-process, and thereby further reduce the loss of P.sub.2 O.sub.5 in the process and further minimize the pollution attributable to wet-process operations. The present invention focuses primarily on converting accumulated acid pond waters, preferably in a setting of wet-processing modified by providing and circulating process water devoid of contaminants so as to supersede polluted acid pond waters.